1. Field of the Invention
The present invention relates to a voltage-controlled oscillator suitable for generation of high-frequency electromagnetic waves (high-frequency signals) such as microwaves and millimeter waves, a high-frequency module including such a voltage-controlled oscillator, and a communication apparatus including such a voltage-controlled oscillator.
2. Description of the Related Art
In general, voltage controlled oscillators for high-frequency signals include a resonant circuit and an active circuit for amplifying a signal output from the resonant circuit, and the resonant circuit includes a resonator having a predetermined resonant frequency, and a variable capacitance diode connected to the resonator for shifting the resonant frequency according to a control voltage.
In the related art, a voltage-controlled oscillator having a parallel connection of a variable capacitance diode and a series-connected circuit including a strip line having a length that is equal to or less than one quarter of the wavelength of the resonant frequency and a capacitor is described in, for example, Japanese Unexamined Patent Application Publication No. 8-288715 and No. 2001-284962.
In such a voltage-controlled oscillator of the related art, for example, the trailing end of the strip line is grounded via the low-impedance capacitor, thus allowing the overall series-connected circuit of the strip line and the capacitor to function as a dielectric circuit, in which the capacitance of the combining circuit of the variable capacitance diode and the series connected circuit varies in inverse proportion to the square of the control voltage. Since the resonant frequency is in inverse proportion to the square root (one-half power) of the capacitance of the combining circuit, the resonant frequency varies linearly with respect to the control voltage, so that the change in linear capacitance with respect to the control voltage is corrected.
Typically, the resonator is connected with the variable capacitance diode using a transmission line such as a microstrip line. When observing the side of the variable capacitance diode from the resonator, the impedance of a combiner of the variable capacitance diode and the transmission line defines the resonant frequency. In the above-described device of the related art, however, the effect of the transmission line is not considered, leading to a problem that the linearity of the resonant frequency does not necessarily increase.
In order to overcome the problems described above, preferred embodiments of the present invention provide a voltage-controlled oscillator, a high-frequency module, and a communication apparatus with improved frequency linearity with respect to a control voltage even when a transmission line is disposed between a resonator and a variable capacitance diode.
According to a preferred embodiment of the present invention, a voltage-controlled oscillator includes a resonant circuit and an active circuit for amplifying a signal output from the resonant circuit, and the resonant circuit includes a resonator having a predetermined resonant frequency, and a variable capacitance diode connected to the resonator for shifting the resonant frequency according to a control voltage.
In the voltage-controlled oscillator, a transmission line is connected between the resonator and the variable capacitance diode, and a capacitive circuit is connected in parallel with the variable capacitance diode.
For example, without the capacitive circuit, the optimum linearity of the resonant frequency is obtained when the length of the transmission line is about one quarter (xcex/4) of the wavelength xcex of the resonant frequency, although the frequency change width is minimized. With the capacitive circuit, in contrast, the optimum linearity of the resonant frequency is obtained when the length of the transmission line is a value greater than one quarter (xcex/4) of the wavelength xcex of the resonant frequency, and the frequency change width increases. Accordingly, the linearity of the resonant frequency can be improved while maintaining the desired frequency change width.
The capacitive circuit may be an open stub defined by another transmission line having one end connected between the variable capacitance diode and the transmission line and the other end which is open.
By setting the length of the open stub to, for example, a value that is equal to or lower than approximately one quarter of the wavelength of the resonant frequency, a desirable capacitive circuit can be achieved.
The capacitive circuit may be a capacitor connected in parallel with the variable capacitance diode. When the capacitive circuit can be a lumped circuit, e.g., when the resonant frequency is low, a single capacitor is connected in parallel, thereby readily providing a desired capacitive circuit.
Preferably, the capacitive circuit has a smaller capacitance than the variable capacitance diode, and the transmission line has a length ranging from about one quarter to about one half of the wavelength of the resonant frequency of the resonant circuit.
With the capacitive circuit connected in parallel with the variable capacitance diode, the optimum linearity of the resonant frequency is obtained when the length of the transmission line is a value that is greater than about one quarter (xcex/4) of the wavelength xcex of the resonant frequency. By setting the length of the transmission line to an appropriate value ranging from about one quarter (xcex/4) to about one half (xcex/2) of the wavelength xcex of the resonant frequency, an impedance change caused by the capacitive circuit can be compensated with the transmission line to optimize the linearity of the resonant frequency and to give a larger frequency change width than the frequency change width without the capacitive circuit.
The resonator may be a dielectric resonator. The dielectric resonator provides a higher Q factor of the overall resonant circuit, resulting in a higher carrier-to-noise (C/N) ratio. However, the resonant frequency change width with respect to the control voltage tends to be low. With the transmission line connected between the resonator and the variable capacitance diode and with the capacitive circuit connected in parallel with the variable capacitance diode, the resonant frequency change width can increase while maintaining the desired frequency linearity. Therefore, the linearity of the resonant frequency, the frequency change width, and the C/N ratio can be improved simultaneously.
In another preferred embodiment of the present invention, a high-frequency module includes the voltage-controlled oscillator according to preferred embodiments of the present invention described above.
In another aspect of the present invention, a communication apparatus includes the voltage-controlled oscillator according to preferred embodiments of the present invention described above.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.